Demographic turnover. A lot of the remaining die-hard Democrats (think union miners and New Deal voters) died between 1990-2005.
That left a large number of the oldest people who might vote Democratic depending on the race due to the state's lean, history and culture, but wouldn't necessarily lean in that direction by default. They made it possible for the kinds of swings we saw in 2000, 2004 & 2008 (strong swing to GOP, strong swing to GOP and Democratic rebound cancelling out demographics, respectively), after which they too mostly died out and the state was left with the modern electorate that scorns Democrats nationally by large margins.
That's not really true, there were many who were ready to vote for Hillary in '08 (and did so in the primary). Check out the AK polling and Southern state polling in general. There were still quite a number of elected southern democrats. Obama destroyed the blue dogs and wrecked that whole contingent of the party.
